1. Field of the Invention
This invention relates to a seeking method for an optimum specific elastic modulus G for a disc substrate used for information recording media such as a magnetic disc (hard disc), an optical disc, etc., to a method for selecting a material suitable for a substrate for information recording media having the optimum specific elastic modulus G given from the seeking method, to a material for providing a substrate for information recording media having the optimum specific elastic modulus G sought in application of the seeking method, to a substrate for information recording media using the material having the optimum specific elastic modulus G, and to a magnetic disc using this substrate. According to this invention, provided are a material for information recording media substrate having the optimum specific elastic modulus G corresponding to a trend of thinner substrates, a substrate for information recording media, and a hard disc.
2. Description of Related Art
Hard disc drivers (HDD) are required these days to increase the recording capacity of the drivers corresponding to higher performances of personal computers. Smaller and thinner disc substrates are also predicted corresponding to reduced sizes and higher performances of personal computers. A disc substrate of 2.5 inches in diameter is expected in future to have a thickness of 0.43 millimeter, or even 0.381 millimeter, from currently 0.631 millimeter. The rotation number of the disc, or the disc speed, is also expected to be higher to 14,000 rpm from currently 7,200 rpm.
As such a disc substrate is made thinner, however, the substrate tends to suffer deflection and warp. Concurrently, flying height of a magnetic head is reduced to increase the recording density, so that such deflection and warp may cause breakdown of the magnetic disc. When a glass substrate that has been used currently for hard disc drivers is made thinner, however, problems due to deflection and warp may become apparent, and the drivers may fail to follow the trend of thinner dimensions. It is also expected that an increased stress will be applied to a substrate under higher-rotational speed. It has not been known whether a substrate currently used is workable under such an increased stress.
To provide substrates fitting trends of smaller, thinner dimensions and higher-rotational speed, new materials need to be developed. However, it is unclear, at this stage, which material having what physical and mechanical characteristics makes disc substrates having a permissible range of deflection, warp, and, further, strength, etc. Although a specific elastic modulus (=Young's modulus/specific gravity) that the substrate material has as an index to deflection and warp of the substrate is known, it is unknown that the substrate material has to have what specific elastic modulus to keep the deflection and warp within the permissible range when the disc size (diameter and thickness) is changed.
It is therefore the first object of the invention to provide a seeking method for an optimum specific elastic modulus G that a material forming a substrate has, from the size of the substrate and a deflection Wmax permissible at a time of rotation, where the substrate is a disc substrate for information recording media such as a magnetic disc (hard disc), an optical disc, etc., and a method for selecting a material suitable for a substrate for information recording media in use of this seeking method.
It is the second object of the invention to provide, in use of the method, a material suitable for a substrate for information recording media meeting trends of smaller and thinner sizes, and higher-rotational speed opertations.
It is the third object of the invention to provide a substrate using the material above and a magnetic disc using such a substrate.